Question

the change in wavelength of light or sound that is due to the relative motion of the source towards or away from an observer.
reflection
doppler effect
radar effect
light effect

Explanation:

The description given is about the change in wavelength of light or sound due to relative motion between the source and the observer, which is the definition of the Doppler Effect. Reflection is about bouncing back of waves, Radar Effect is not a standard term for this phenomenon, and Light Effect is too vague and not related to this specific concept.

Answer:

B. Doppler Effect